The invention relates to a composite junction or joint for integration into a sandwich panel so as to permit the fitting of at least one external member to said panel.
In particular, the composite joint according to the invention is designed for permitting the transmission of high static and/or dynamic stresses between a sandwich panel and one or more members fixed to said panel.
The invention also relates to a sandwich panel integrating one or more composite joints of this type.
A preferred application of the invention is in the aeronautical and space industries. However, it can be used in other technical fields without passing outside the scope of the invention.
Sandwich panels are used in numerous industries, particularly due to their lightness and their considerable rigidity, particularly flexural rigidity.
A sandwich panel is formed from a cellular core and two covering layers positioned on either side thereof. The cellular core is generally in the form of a honeycomb or foam structure. This configuration leads to a significant weight gain compared with a homogeneous structure having the same rigidity. As a function of the intended use, the covering layers and the cellular core are metallic and/or composite and/or synthetic.
It is known in the aeronautical industry to use sandwich panels for e.g. forming the floors of cabins, internal partitions and wing elements. Such panels are also used in the space industry, where the weight reduction requirement is imperative. High stresses are then transmitted to the structures during the take-off phases. In addition, high thermal stresses are present, due to the high temperature gradient existing between the shady areas and the sunny areas.
It is also known that a composite material partly formed from resin gives off into space the water present in its structure, which has the effect of causing displacements liable to modify the position of instruments or equipments mounted on the panel.
When a random external member is mounted on a sandwich panel, the joint must be such that it transmits to the coverings and to the core of the panel stresses of all types without giving rise to a fracture or permanent deformations. Moreover, the service life of the joint must be compatible with the sought service life for the panel.
To produce such a joint, one known solution consists of integrating into the panel a monolithic, connected part of a random shape, which is normally made from metal and which is known as a insert. Said insert is dimensioned so as to receive the stresses and spread them to the panel.
Inserts conventionally used for ensuring the junction between an external member and a sandwich panel may either pass completely through the panel or may be non-issuing. In the latter case, the insert is generally included in the panel prior to the bonding of the coverings to the cellular core. When the insert traverses the panel, it is generally placed in a cavity machined in at least one of the coverings and in the core of the panel. The fixing of the insert in the cavity is then ensured by a solidified material such as a resin or a foaming film. In exemplified manner, U.S. Pat. Nos. 5,240,543, 5,378,099, FR-A-1 243 582, FR-A-1 132 264 and FR-A-2 452 021 relate to different types of inserts completely traversing a sandwich panel.
When this conventional procedure is used, the mechanical performance characteristics of the inserts can be optimized by giving them an external contour for fastening the resin. In addition, the dimensions of the cavity in which the insert is housed take account of the shape thereof and the stresses transmitted through the joint or junction. Thus, the perimeters of the cavity and the insert are chosen as a function of shear stresses, which must be transmitted by the interface between the insert and the panel, in order that the stress level remains acceptable for the materials used.
This conventional joining procedure suffers from a certain number of disadvantages.
Thus, the respecting of the different parameters referred to hereinbefore generally leads to the use of inserts having relatively large dimensions, whose weight, increased by that of the fixing resin, significantly increases the weight of the sandwich panel. This phenomenon is particularly sensitive when a single insert integrates several fixing points.
Moreover, the area materialized by the insert has different physical properties from those of the remainder of the sandwich panel. The resulting discontinuity gives rise to a distortion in the deformations of the panel, particularly in the case where the latter is subject to thermal stresses. This discontinuity is particularly prejudicial in the case of a large, monolithic insert.
Sandwich panels are also known, whose cellular core is formed from juxtaposed, tubular cells constituted by braided fibre roves, as disclosed in the document xe2x80x9cComposite Airframe Structurexe2x80x9d published in September 1995, pp 270/271. In such a panel the cells are juxtaposed in the longitudinal direction, i.e. parallel to the panel covering layers and form the entire panel core. A panel constructed according to said document is only able to receive a load uniformly distributed over its whole surface.
The object of the invention is a composite junction or joint, whose original configuration enables it to ensure the progressive transmission of stresses between a relatively rigid, external member and a significantly less rigid, cellular core of a sandwich panel, whilst giving a considerable dimensional stability in the presence of thermal and/or hygroscopic stresses.
According to the invention, this result is obtained by means of a composite joint for the fitting of at least one external member to a sandwich panel comprising a cellular core and two covering layers placed on either side thereof, said joint being characterized in that it comprises a plurality of juxtaposed, elementary, tubular cells orientable in the thickness direction of the panel within a recess formed in the core of the sandwich panel and at least one stress introduction cell in which is housed at least one rigid part for fixing said external member, each stress introduction cell being separated from a peripheral edge of the joint, in all directions, by at least one elementary, tubular cell.
In a composite joint designed in this way, it is possible to adapt the cross-section and size of the tubular cells, their number and thickness, as well as the orientation of the partitions as a function of the intensity and orientation of the stresses which have to be transmitted between the rigid part and the cellular core of the panel, e.g. in such a way that the rigidity of the joint evolves progressively between the rigid part and the core of the panel. By comparison with a conventional, one-piece insert used under the same conditions, the composite joint according to the invention is placed in a cavity having essentially the same dimensions. The rigid part, which can in particular be metallic, is consequently much smaller and therefore less heavy than a conventional insert. Thus, the composite joint according to the invention leads to a weight gain, which can reach approximately 50% as a function of the particular application.
In a preferred embodiment of the invention, each of the elementary, tubular cells comprises a composite wall formed from braided fibre roves and resin. The elementary, tubular cells, which form most of the composite joint, can therefore be produced from a combination of intrinsically stable materials (fibres, e.g. of carbon, which are thermally stable and resin, which is stable from the water-solidification standpoint). Moreover, the consequences of a possible desorption in vacuo are very limited, because the joint contains a small resin volume compared with the fibre volume and a resin having a high hygroscopic stability can be chosen. The resulting composite joint consequently has a dimensional stability far superior to that of a conventional insert.
In the preferred embodiment of the invention, the juxtaposed, elementary, tubular cells are separated by partitions, each comprising two composite walls interconnected by resin. In order to arrive at this result, it is possible to manufacture the joint using resin transfer moulding or RTM. Dry fibre preforms corresponding to the elementary, tubular cells are then placed in a mould. The joint is obtained by injecting a resin under pressure into the mould and then polymerizing said resin.
To ensure an effective stress transmissions, the partitions separating the juxtaposed, elementary, tubular cells are aligned in at least two stress transmission directions.
Preferably, the separating partitions of the juxtaposed, elementary, tubular cells form planes orientable perpendicular to the covering layers of the sandwich panel.
In a non-limitative, special embodiment according to the invention, at least some of the elementary, tubular cells have a square cross-section.
In this case, the elementary, tubular cells having a square cross-section, whereof a diagonal passes through a stress introduction cell, can advantageously comprise an internal partition oriented in accordance with said diagonal.
The rigid part, which can be metallic or made any other appropriate material is connected in preferred manner to the stress introduction cell by resin. When the joint is produced by the RTM method, the rigid part is then integrated into the joint during moulding.
The external member can be fixed to the rigid part either directly, or by means of an appropriate, randomly shaped, connecting part. In all cases, the rigid part can comprise a fixing hole oriented in a longitudinal direction with respect to the stress introduction cell.
The invention also relates to a sandwich panel incorporating one or more composite joints of this type.